Making your office 2.0
Ismael Ghalimi has compiled a list of web based productivity software with which you could potentially bring your whole office software suite to the web; Office 2.0 Database.
Of this list i actively use DabbleDB (CRM and list manager), Linkedin (contacts) and (as soon as they fix some issues) Blinksale (invoicing) for my own business. And Gmail (webmail), Del.icio.us (bookmarks), Pixoh (photo editing), Flickr (photo sharing) and Wordpress (blogging) personally.